Rechargeable prismatic batteries are used in a variety of industrial and commercial applications such as fork lifts, golf carts, and uninterruptable power supplies. Many electric vehicles presently in the planning stages use prismatic batteries.
Rechargeable lead-acid batteries are the most widely used type of battery. Lead-acid batteries are an excellent power source to drive a starter motor for an internal combustion engines. However, lead acid batteries have an energy density of only 30 Wh/kg and in an electric vehicle are capable of providing a vehicle range of only 30 to 120 miles before requiring a recharge. In addition, lead acid batteries require 6 to 12 hour to recharge, and contain large quantities of toxic materials. Further, electric vehicles using lead-acid batteries have sluggish acceleration, top speeds of only 50 to 60 mph, and a lifetime of 20,000 miles.
Nickel metal hydride batteries ("Ni--MH batteries") are far superior to lead acid batteries. Ni--MH prismatic batteries are also the most promising type of battery available for electric vehicles. For example, Ni--MH batteries, :such as those described in copending U.S. patent application Ser. No. 07/934,976 to Ovshinsky et al., now U.S. Pat. No. 5,277,999, the contents of which are incorporated by reference, have an energy density of 80 Nh/kg, can power a vehicle over 300 miles before requiring recharging, can be recharged in less than one hour, and contain no toxic materials. Prototype electric vehicles using Ni--MH batteries have an acceleration of 0-60 in 8 seconds, a top speed of 90 mph, and a projected lifetime of more than 100,000 miles.
Ni--MH batteries use a nickel hydroxide positive electrode and a hydrogen storage negative electrode. The electrodes are separated by a non-woven, felted, nylon or polypropylene separator. The electrolyte is usually an alkaline electrolyte, for example, containing 20 to 45 weight percent potassium hydroxide.
Ni--MH batteries were previously classified based on whether they used AB.sub.2 or AB.sub.5 alloys as the hydrogen storage material of the negative electrode. Both types of material are discussed in detail in copending U.S. patent application Ser. No. 07/934,976, referenced above. The distinction between AB.sub.2 and AB.sub.5 alloys have disappeared as the formulations of each type become based on multi-elemental substitution and disorder.
By forming metal hydride alloys from such disordered materials, Ovshinsky and his team were able to greatly increase the reversible hydrogen storage characteristics required for efficient and economical battery applications, and produce batteries capable of high density energy storage, efficient reversibility, high electrical efficiency, efficient bulk hydrogen storage without structural changes or poisoning, long cycle life, and repeated deep discharge. The improved characteristics of these alloys results from tailoring the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix. Disordered metal hydride alloys have a substantially increased density of catalytically active sites and storage sites compared to single or multi-phase crystalline materials. These additional sites are responsible for improved efficiency of electrochemical charging/discharging and an increase in electrical energy storage capacity. The nature and number of storage sites can even be designed independently of the catalytically active sites. More specifically, these alloys are tailored to allow bulk storage of the dissociated hydrogen atoms at bonding strengths within the range of reversibility suitable for use in secondary battery applications.
Some extremely efficient electrochemical hydrogen storage materials were formulated, based on the disordered materials described above. These are the Ti--V--Zr--Ni type active materials such as disclosed in U.S. Pat. No. 4,551,400 ("the '400 Patent") to Sapru, Hong, Fetcenko, and Venkatesan, the disclosure of which is incorporated by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the '400 Patent utilize a generic Ti--V--Ni composition, where at least Ti, V, and Ni are present with at least one or more of Cr, Zr, and Al. The materials of the '400 Patent are multiphase materials, which may contain, but are not limited to, one or more AB.sub.2 phases with C.sub.14 and C.sub.15 type crystal structures.
Other Ti--V--Zr--Ni alloys are also used for rechargeable hydrogen storage negative electrodes. One such family of materials are those described in U.S. Pat. No. 4,728,586 ("the '586 Patent") to Venkatesan, Reichman, and Fetcenko, the disclosure of which is incorporated by reference. The '586 Patent describes a specific sub-class of these Ti--V--Ni--Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. The '586 Patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them.
In contrast to the AB.sub.2 alloys described above, the AB.sub.5 alloys were generally considered "ordered" materials that had different chemistry, microstructure, and electrochemical characteristics compared to the AB.sub.2 alloys. However, while this appears to have been true for the early AB.sub.5 alloys, it is not true for the more recently developed ones.
The performance of the early ordered AB.sub.5 materials was poor, but in the early 1980's, as the degree of modification increased (that is as the number and amount of elemental modifiers increased), their performance began to improve significantly. This is due as much to the disorder contributed by the modifiers as it is to their electrical and chemical properties. This evolution of AB.sub.5 type alloys from a specific class of "ordered" materials to the current multicomponent, multiphase "disordered" alloys that are very similar to AB.sub.2 alloys is shown in the following patents: (i) U.S. Pat. No. 3,874,928; (ii) U.S. Pat. No. 4,214,043; (iii) U.S. Pat. No. 4,107,395; (iv) U.S. Pat. No. 4,107,405; (v) U.S. Pat. No. 4,112,199; (vi) U.S. Pat. No. 4,125,688 (vii) U.S. Pat. No. 4,214,043; (viii) U.S. Pat. No. 4,216,274; (ix) U.S. Pat. No. 4,487,817; (x) U.S. Pat. No. 4,605,603; (xii) U.S. Pat. No. 4,696,873; and (xiii) U.S. Pat. No. 4,699,856. (These references are discussed extensively in U.S. Pat. No. 5,096,667 and this discussion is specifically incorporated by reference).
Simply stated, in the AB.sub.5 alloys, like the AB.sub.2 alloys, as the degree of modification increases, the role of the initially ordered base alloy is of minor importance compared to the properties and disorder attributable to the particular modifiers. In addition, analysis of the current multiple component AB.sub.5 alloys indicates that current AB.sub.5 alloy systems are modified following the guidelines established for AB.sub.2 systems. Thus, highly modified AB.sub.5 alloys are identical to AB.sub.2 alloys in that both are disordered materials that are characterized by multiple components and multiple phases and there no longer exists any significant distinction between these two types of multicomponent, multiphase alloys.
In electric vehicles, weight of the batteries is a significant factor because the largest component of the total weight of the vehicle is the weight of the batteries. For this reason, reducing the weight of individual batteries is a significant consideration in designing batteries to power electric vehicles. One method to reduce weight for prismatic batteries for electric vehicles is to use plastic to replace metal components such as the case and parts of the electrodes.
Plastic cases are extensively used for lead acid batteries. Adapting this technology or other plastics technology to produce large, light cases for prismatic Ni--MH hydride electric vehicle batteries has not been difficult. Similarly, the use of pasted electrodes constructed by spreading alloy powder and a binder on a plastic screen or film as a means of reducing weight is also well known.
Heat generated during charging and discharging Ni--MH batteries is normally not a problem in small consumer batteries or even in larger batteries when they are used singly for a limited period of time. For example heat is rarely a problem with lead acid automobile batteries used to start internal combustion engines. But, large batteries that serve as a continual power source, particularly when more than one is used in series or in parallel, such as in a satellite or an electric vehicle, do generate sufficient heat on charging and discharging to affect the ultimate performance of the cell.
The prior art suggests a variety of solutions to excess heat: U.S. Pat. No. 3,830,663, to Eisele, et al., describes a battery holder for providing a thermal path from the battery to the skin of a satellite. The holder involves top and bottom plates of anodized material that are in contact with the top and bottom of the battery and painted black to radiate absorbed heat to the skin of the satellite. This patent contains no teaching at all regarding heat transfer from the interior of the battery to the battery case.
U.S. Pat. No. 4,115,630 to Van Ommering, et al., describes a metal oxide-hydrogen battery having bipolar electrodes arranged in a centrally drilled stack. This patent describes conducting heat generated in the electrode stack via the hydrogen gas of the cell. In particular, this patent specifically states that heat conduction perpendicular to electrode plates is 10-20 times smaller than conduction parallel to electrode plates, so that cells using flat electrodes must be modified significantly which adds unacceptably weight. Thus, this patent teaches away from cells using flat plate electrodes.
J. Lee, et al. describe resistive heating and entropy heating in lead-acid and nickel/iron battery modules in 133(7) JESOAN 1286 (July, 1986). This article states that the temperature of these batteries is due to resistive heating and entropy changes of the electrochemical reactions often varies considerably during their operation. This article notes that the thermal resistance caused by the cell case plays an important role as the cell temperature increases. While this reference suggests that an additional "cooling structure" should be added to the battery, no suggestion is made as to the specifics of such a structure. Further, there is no teaching or suggestion that the interior components of the battery might play any role in thermal management.
U.S. Pat. No. 4,865,928 to Richter describes a method of removing heat from the interior of a high-performance lead acid battery by attaching a U-shaped tube to the negative electrode grid and circulating a coolant through the tube. This patent contains no teaching or suggestion that the interior components of the battery might play any role in thermal management.
U.S. Pat. No. 5,035,964 to Levinson et al. describes a finned heat sink connected to a battery by a copper or aluminum bar, where the finned heat sink produces a convective flow of air in a chimney to cool the battery. This patent contains no teaching or suggestion that the interior components of the battery might play any role in thermal management.
The solutions suggested in these references all involve the addition of some kind of apparatus to hasten the dissipation of excess heat. The addition of apparatus results in added weight and cost as well as reduced efficiency. Because battery weight is one of the greatest problems confronting EV manufacturers, anything that increases weight does not constitute a solution to the problem. In addition, as noted above, none of these references contain any teaching or suggestion that the interior components of the battery could play a role in thermal management of the cell.
While the use of plastic may overcome the weight problem, plastic is an inefficient conductor of heat. In addition, in sealed cells, the ability of the case to withstand high internal pressure is important. This is particularly true in NI-MH cells, where the cells normally must be capable of containing gas generated on overcharge and overdischarge without venting, or deformation of the cell case.
The operation of a nickel metal hydride cell produces gas during overcharge and overdischarge. As a result, the internal cell pressures may vary substantially during operation. Because of this, hydrogen storage cells are typically produced as either sealed cells or vented cells. During normal operation, a sealed cell does not permit the venting of gas to the atmosphere. In contrast, a vented cell will release excess pressure by venting gas as part of its normal operation. As a result of this difference, the vent assemblies used in sealed and vented cells are quite different from one another, and the amounts of electrolyte in the cell container relative to the electrode geometry differ significantly.
Sealed cells are manufactured predominantly in cylindrical and rectangular configurations. Sealed cells are usually designed to operate in a starved electrolyte configuration. The cell enclosure for a sealed cell is normally metallic and designed for operation at pressures up to about 100 pounds per square inch absolute or even higher. Sealed cells are characterized by the substantial absence of any required maintenance.
As discussed above, the operation of a NiMH cell produces gases depending on the amount of electrolyte, the operating temperature, as well as variations in components, chemical concentrations, and manufacturing techniques. The production of these gases frequently result in deformation of the cell can. It is, of course, desirable that such deformation be avoided in large cell packs.